Porous silicon porosity

Figure 2. Effective medium approximations relating porous silicon porosity to refractive index for a wavelength of 1.55 /am. The differences arise from different assumptions made about the porous silicon morphology.

Fig. 4 SEM images of netiroblastoma cells cultured on a porous silicon porosity gradient. The cells display different morphology dependent upon die pore size, (a) 1,000-3,000 nm pores, (b) 300-1,000 nm. (c) 50-100 nm and (d) 5-20 nm (Khung et al. 2008)...

The Values of the KWW Exponent v, Fractal Dimension Dp, and Porosity m Obtained from Relative Mass Decrement Measurements and Average Porosity ( p) Estimated from Dielectric Spectra for Porous Silicon Samples of 20- and 30-pm Thickness [2]... 

There are essentially three different ways how to prepare nanometer sized silicon particles. The porous silicon is, as already mentioned, prepared by anodic etching of silicon wafers in an HF/ethanol/water solution [6, 7]. The microporous silicon has typically a high porosity of 60-70 vol.%, and it consists of few nm thin wires which preserve the original orientation of the wafer. The thickness of the wires varies within the PS layer and the material is very brittle. Free standing PS films can be prepared by application of a high current density after the usual etching of the desired thickness of the PS. 

Non-oxide ceramic materials such as silicon carbide has been used commercially as a membrane support material and studied as a potential membrane material. Silicon nitride has also the potential of being a ceramic membrane material. In fact, both materials have been used in other high-temperature structural ceramic applications. Oxidation resistance of these non-oxide ceramics as membrane materials for membrane reactor applications is obviously very important. The oxidation rate is related to the reactive surface area thus oxidation of porous non-oxide ceramics depends on their open porosity. The generally accepted oxidation mechanism of porous silicon nitride materials consists of two... 

Figure 2. Flash diameter vs pore wall thickness for difFerent porosity in porous silicon impregnated with NaClOi. Porous silicon thickness was 30 pm.

Continuous Pt films at the surface of porous silicon cannot be applicable as catalytic coats for fuel cells electrodes. Quite the contrary, Pt coats should save its porosity to allow an easy penetration of gaseous iuel and to have the effective surface area as high as possible. So, the layers of electrodeposited Pt of about 100 nm in thickness, as illustrated in Fig. Ic, are optimal catalytic Pt films for micro fuel cell electrodes. 

R. Herino, G. Romchil, K. Boala, and C. Bertrand, Porosity and pore size distribution of porous silicon layers, J. Electrochem. Soc. 134, 1994, 1987. 

D. Xu, G. Guo, L. Gui, Y. Tang, B. Zhang, and G. Qin, Preparation and characterization of freestanding porous silicon films with high porosities, Electrochem. Solid-State Lett. 1(5), 227, 1998. 

Porous silicon (PS) is one of the nanoscale modifications of silicon. There are various approaches to PS producing that are now in use. The technique most generally employed today is known as wet anodization of a crystalline silicon. With this technique, yield parameters of porous material (porosity, pore size and shape, interpore distance) may be readily varied by anodization regimes. However, it is well known the problem of the PS stability influencing the physical properties of the PS layers. P S instability is c onditioned b y very large specific surface area of the porous material. 

Kanungo, X, Saha. H. and Basu S. Effect of porosity on the performance of surface modified porous silicon hydrogen sensors , (2010b) Sensor. Actual.B. 147, 145-51. 

Finally, silicon processing techniques have also been leveraged to make novel ion conducting membranes. Porous silicon can be readily created in a silicon substrate by an anodization process in HF. The result is a nanoporous structure with over 50 % porosity. By forming thick membranes of porous silicon (>40 pm) loaded with sulfonic acid, ionic conductivities similar to Nafion were attained (10-10 S/cm) [5]. 

Figure 1. Plan view scanning electron microscopy image of porous silicon. The dark, circular regions are the air pores and the interconnected brighter region is the silicon matrix. The pore size and porosity can be changed by modifying the formation conditions and silicon doping.

Figure 4. The response of the porous silicon waveguide sensor upon exposure to biomolecules depends on the percent of the optical field interacting with the molecules. The inset shows the field distribution in the waveguide with the majority of the field inside the porous silicon (LP = low porosity layer, HP = high porosity layer) where biomolecules may be captured. Therefore, even if only a few biomolecules are present in the waveguide, they can be detected...

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